Present communication systems currently employ pseudo-noise (PN) spread spectrum modulation. The purpose of the spread spectrum modulation depends on the particular application. Such communication systems can be utilized for purposes of security, anti-jam protection, multiple access capability, or a safe level of power flux density radiated back to earth. Regardless of the application, the problems of acquiring and tracking the PN code are paramount in all these systems. In order to optimize a system, the code is to be acquired in as short a time as possible.
In communications systems utilizing pseudo-noise sequences or codes in their signals, it is a requisite that the phase of the receiver's code sequence be synchronized to that of the transmitted sequence. It is therefore the purpose of a code tracker within a spread spectrum receiver to seek out this phase and follow it by providing the appropriately phased replica of the code to the decoder unit within the spread spectrum receiver. When the incoming coded signal is correlated with the appropriately phased replica at the receiver, the information on the transmitted signal may be read out.
As used herein, a spread spectrum system is one in which the transmitted signal is spread over a wide frequency band, much wider in fact than the minimum band width required to transmit the information being sent. Three general types of modulations produce spread spectrum signals, e.g. (1) Modulation of a carrier by a digital code sequence whose bit rate is much higher than the information signal bandwidth. Such systems are called "direct sequence" modulated systems; (2) Carrier frequency shifting in discrete increments in a pattern dictated by a code sequence. These are called "frequency hoppers." The transmitter jumps from frequency to frequency within some predetermined set, with the order of frequency usage being determined by a code sequence; and (3) Pulsed-FM or "chirp" modulation in which a carrier is swept over a wide band during a given pulse interval.
The term "pseudo-noise" mentioned above refers to a predetermined apparently random pulse sequence having a recurring period or cycle which is long compared with a prevailing information or message duration. This pseudo-random pulse sequence is usually used in a direct sequence system which, in the biphase phase shift keyed embodiment, involves a carrier which is 180.degree. phase-shifted in accordance with the output of a pseudo-random-number code generator. Thus the incoming signal consists of a sequence of phase transitions from one constant value to another. In one system, these transitions occur at a fixed frequency, with the period of the transitions being referred to as one "chip." Of particular interest in many applications is the binary, pseudo-random-number code which consists of a pseudo-randomly generated sequence of numbers having a value of +1 or -1. A pseudo-random-number code is one which is derived from a sequence which can be generated systematically but which has some of the properties of a random-number sequence. Pseudo-random codes are well known and are of practical interest since a receiver which is capable of generating the pseudo-random-number code sequence can lock onto a pseudo-random-number code signal which looks to other receivers like noise. At the same time, spurious signals which may accompany the incoming pseudo-random-number code signal (as, for example, spurious signals generated by thermal noise or external interfering signals) will appear as noise to the receiver and may be rejected by proper filtering techniques. Since pseudo- randomly coded spread spectrum signals look like noise when received by a conventional receiver, the class of receivers which detect spread spectrum pseudo-random-number codes are called pseudo-noise (PN) spread-spectrum receivers.
U.S. Pat. Nos. 3,305,636; 3,350,644; 3,402,265; 3,439,279; 3,629,505; 3,666,889; 3,852,354; 4,007,330; 4,017,798; 4,039,749; 4,048,563; 4,092,601; 4,122,393; 4,203,071; 4,214,209; and 4,221,005 described various correlator systems for providing the appropriately phased replica of the PN code to be used in the decoders of the spread spectrum receivers.
For parallel-correlator systems, a bank of correlators is fed with the incoming signal, with each correlator channel being provided with progressively advanced and retarded versions of a local PN code generator sequence. This means that each correlator channel is provided with a differently phased replica of the predetermined pseudo-noise code. Were it not for noise in the system, the correlator channel having the highest correlation output would indicate which of the differently phased replicas is the one which matches the phase of the transmitted code. However, Doppler shifts, propagation disturbances and interference result in more than one correlator channel having a high correlation value. In order to determine which correlator channel is the one identifying the appropriately phased replica, various delay-locked loop and integration or averaging techniques are utilized. While all of the prior art signal acquisition and tracking systems can acquire and lock up to the appropriately phased replica, excessive lock-up time precludes the use of these systems where the phase of the incoming signal rapidly changes. This problem is particularly severe in communication with fast-moving vehicles such as jet aircraft, rockets, and nonsynchronous satellites.
In the past, fixed weighting systems have been utilized in which each individual correlator channel is provided with a predetermined weight depending upon certain a priori considerations, such as slant range to the transmitting satellite, Doppler shift, and known atmospheric effects. From this a priori information, it can be ascertained which channel or channels have a high probability of being those channel or channels associated with the correctly phased replica. Once having ascertained these channels with a priori knowledge, their outputs may be given increased weights, whereas other channels are given decreased weights. However, all fixed weighting systems suffer from non-adaptive assigning of weights.
In an attempt to decrease signal acquisition time of the fixed weighting systems, the sharpness of the detector characteristic is adjusted depending on the noise conditions and the probability distribution of signal phase. If relatively little knowledge is available with respect to which probabilities can be calculated, the detector characteristic is relatively flat to permit signal acquisition by a large number of correlator channels. This is equivalent to extending the acquisition range of the receiver by extending the detector response. As more information becomes available as to the incoming signal, the detector characteristic is narrowed so as to effectively reduce the number of correlator channels and thus the receiver range. Change of detector range can be accomplished by adding or decreasing shift register bits for the shift register used to generate the phased replicas. Alternatively this can be accomplished by changing the correlation channel weights making up the nonlinear detector characteristic. This latter type of system is described in detail in the aforementioned U.S. Pat. No. 4,203,071 issued May 13, 1980 to W. M. Bowles, D. B. Cox, Jr., and W. J. Guinon, assigned to the assignee thereof and incorporated herein by reference. This patent describes a detection and tracking system which permits rapid acquisition but does not involve adaptive tracking or combined adaptive acquisition and adaptive tracking since it relies solely on statistical methods of computing error. In short, no signal sampling is used to automatically adjust weights. Moreover, the weights are not adjusted to enhance the weight of a channel which is established as having a high correlation value.
One of the aforementioned patents, U.S. Pat. No. 4,007,330, describes a system for accommodating Doppler shifts by a correction system which selects among three correlator channels according to correlation. Here the incoming signal is delayed by different amounts and then correlated with a predetermined signal, with the correlator channel having the peak correlation identifying the Doppler shift. It will however be appreciated that this system does not utilize adaptive weighting.